add missed links (Azure#19509)

Author: zhichengliu12581

sdk/appconfiguration/spring-cloud-starter-azure-appconfiguration-config/README.md

@@ -185,7 +185,7 @@ public class MyCredentials implements AppConfigCredentialProvider, KeyVaultCrede
 
 #### Client Builder Customization
 
-The service client builders used for connecting to App Configuration and Key Vault can be customized by implementing interfaces `ConfigurationClientBuilderSetup` and `SecretClientBuilderSetup` respectively. Generating and providing a `@Bean` of them will update the default service client builders used in [App Configuration SDK](https://github.com/Azure/azure-sdk-for-java/tree/master/sdk/appconfiguration/azure-data-appconfiguration#key-concepts) and [Key Vault SDK](https://github.com/Azure/azure-sdk-for-java/tree/master/sdk/keyvault/azure-security-keyvault-secrets#key-concepts). If necessary, the customization can be done per App Configuration store or Key Vault instance.
+The service client builders used for connecting to App Configuration and Key Vault can be customized by implementing interfaces `ConfigurationClientBuilderSetup` and `SecretClientBuilderSetup` respectively. Generating and providing a `@Bean` of them will update the default service client builders used in [App Configuration SDK][app_configuration_SDK] and [Key Vault SDK][key_vault_SDK]. If necessary, the customization can be done per App Configuration store or Key Vault instance.
 
 ```java
 public interface ConfigurationClientBuilderSetup {
@@ -242,3 +242,7 @@ public class MyClient implements ConfigurationClientBuilderSetup, SecretClientBu
 [enable_managed_identities]: https://docs.microsoft.com/azure/active-directory/managed-identities-azure-resources/overview#how-can-i-use-managed-identities-for-azure-resources
 [support_azure_services]: https://docs.microsoft.com/azure/active-directory/managed-identities-azure-resources/services-support-managed-identities
 [spring_conversion_duration]: https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html#boot-features-external-config-conversion-duration
+[azure_subscription]: https://azure.microsoft.com/free
+[maven]: https://maven.apache.org/
+[app_configuration_SDK]: https://github.com/Azure/azure-sdk-for-java/tree/master/sdk/appconfiguration/azure-data-appconfiguration#key-concepts
+[key_vault_SDK]: https://github.com/Azure/azure-sdk-for-java/tree/master/sdk/keyvault/azure-security-keyvault-secrets#key-concepts